The present invention is in the field of adaptive optical systems, and in particular, relates to the control of electromagnetic fields propagated from a transmitting telescope to a receive telescope through a turbulent medium.
Adaptive optical systems are an enabling technology for a wide range of applications. Adaptive optical systems provide a means to compensate for the amplitude and phase fluctuations that result from electromagnetic wave propagation through a turbulent medium. Adaptive optical systems utilize a wavefront sensor to measure the amplitude and phase distortions observed on a wavefront sensing beacon and use these measurements to apply a set of phase commands to one or more phase correction devices to compensate for the effects of turbulence on propagation of electro-magnetic waves. These systems have seen great success in astronomical telescopes, obtaining nearly diffraction-limited images for large (greater than 3-meter aperture diameter) telescopes. These successes, however, have been achieved at high elevation angles (within 60xc2x0 of zenith), where only the phase fluctuations that result from propagation through turbulence are significant. In this case, only a single-phase correction device, typically a continuous facesheet deformable mirror, is required to compensate for the phase fluctuations.
At lower elevation angles, and in horizontal path applications, the amplitude fluctuations and anisoplanatic considerations become significant. Horizontal path applications, such as the U.S. Air Force""s Airborne Laser Program and, in the commercial arena, free space optical communications and quantum cryptography, have become popular recently. These applications have encouraged the development of advanced adaptive optical systems that can compensate for both amplitude and phase fluctuations that result from propagation through a turbulent medium (See pending application Ser. No. 09/682,146 filed Jul. 26, 2001, now U.S. Pat. No. 6,452,146, entitled, xe2x80x9cElectro-Optical Field Conjugation Systems,xe2x80x9d and herein incorporated by reference and Barchers, J. D., xe2x80x9cClosed loop stable control of two deformable mirrors for compensation of amplitude and phase fluctuations,xe2x80x9d submitted for publication in the Journal of the Optical Society of America A., July, 2001.) Power beaming to aerospace platforms at low elevation angles is another subject of recent interest.
In such applications as power beaming and horizontal path optical communication, an adaptive optical system in the transmitting telescope is used to pre-compensate a coherent laser beam to maximize the power collected by a receive telescope. It has been widely assumed that the optimal wavefront-sensing beacon for use in measuring the effects of atmospheric turbulence is a point source beacon. This bias is largely due to the fact that state of the art adaptive optical systems, such as that under development for the Airborne Laser Program, have largely been developed by the military industrial complex in which the objective of an adaptive optical system is to maximize the power density in a very small area on a target at a long range. For applications in which the objective is to maximize the received power in a collecting bucket (i.e., aperture), it is not obvious that this is the correct approach. The present invention defines the optimal approach to maximize received power in a collecting aperture and establishes that in fact, the optimal approach is not unreasonable to implement.
In a preferred embodiment, the invention provides a means for maximum transmission of power between two telescopes. The invention requires that each telescope have an adaptive optical system capable of correcting at least the phase errors that result from propagation through a turbulent medium, although correction of both amplitude and phase errors is preferred. Maximum power transmission is achieved by correcting the observed phase aberrations in each telescope and applying these same corrections to the beam propagated towards the respective xe2x80x9cotherxe2x80x9d telescope. This leads to a natural iteration, which converges towards a solution that maximizes the received power in each telescope. The significant benefit is the reduction of the required aperture size of each telescope by a factor of more than 100 when compensation of both amplitude and phase fluctuations is used on both telescopes.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, illustrating by way of example the principles of the invention.